Display devices for displaying results of processing executed by information processors (for example, liquid crystal display devices etc.) are becoming very important with the spread of information processors such as personal computers.
At present electrochromic display devices utilizing so-called electrochromism are proposed as new display devices (refer to, for example, M. Gratzel, Nature, 409 (2001) 575). The electrochromism refers to the reversible color change (a colored state and decolored state) of electrochromic materials, during the electrochemical redox reaction. Electrochromic devices have such advantages as that (1) they show superior visibility, (2) they can provide a large display, (3) they have low viewing angle dependency, and (4) they can display clear images. In these respects they are expected to be applied especially to extremely thin displays such as so-called electronic papers.
Viologen compounds are known as materials used for display devices utilizing the electrochromism (refer to, for example, Japanese Unexamined Patent Application Publication No. 2009-86259 and C. L. Bard, A. T. Kuhn, Chem. Soc. Rev., 10 (1981) 49). Viologen compounds generally show remarkable change in color between a colored state and a decolored state. Therefore viologen is expected to become a key material in developing electrochromic display devices.
However, viologen compounds present purple color in color-developed state, and will not present white-color that is a basic color of display devices. Thus, any electrochromic materials which present a white color state and a decolorized state, have not been obtained so far.